is a professor of ophthalmology and professor of visual science at the University of Rochester. He specializes in refractive corneal and premium intraocular lens surgery. Dr. MacRae’s research interests include the optics of corneal and intraocular lens surgery. He also has a strong interest in the surgical correction of presbyopia. He has been Senior Associate Editor of the Journal of Refractive Surgery and serves on the American Academy of Ophthalmology – FDA Advisory Committee on Multifocal and Accommodating IOL’s.
received his PhD degree from Osaka University in Japan in 1998 in High Power Laser Optics and is Professor of Ophthalmology, the Institute of Optics, Center for Visual Science and Biomedical Engineering at the University of Rochester. His research focus is on visual and physiological optics, understanding optical quality of the eye, its impact on spatial vision and advanced correction of the ocular aberration using customized ophthalmic lenses, adaptive optics and refractive and cataract surgery. His recent research includes investigating neural adaptation in highly aberrated eyes, presbyopia correction technology, high-resolution ocular imaging and emmetropization.
13.1 Introduction
An understanding of the through-focus image quality and optical characteristics of presbyopia-correcting IOLs offers the clinician the ability to properly set patients’ expectations. As more presbyopia-correcting IOLs become available, it is increasingly important for the clinician to have an understanding of their features and the challenges facing patients using these lenses. Optical bench testing of IOLs offers an ability to objectively assess through-focus image quality. Furthermore, incorporating adaptive optics into optical bench testing allows quantification of the impact of corneal aberrations on through-focus image quality of presbyopia-correcting IOLs.
13.2 Optical Bench System
The most straightforward method of assessing image quality of an IOL is to directly capture the retinal image of a resolution target through the IOL in a model eye on an optical bench. To assess pseudophakic image quality in real-life conditions, corneal lower and higher order aberrations were induced in the model eye using adaptive optics.
Our lab has developed the world’s first adaptive optics (AO) IOL metrology bench not only to assess the through-focus image quality of presbyopia-correcting IOLs but also to determine the influence of corneal aberrations on these IOLs. A simplified schematic of the system is shown in Fig. 13.1. For more details, see Zheleznyak et al. [1]. The main features of an AO-IOL metrology bench are (1) a model eye to house the IOL, (2) an AO device for corneal aberration induction, and (3) an imaging device to capture the retinal image.
Fig. 13.1
Simplified schematic of adaptive optics IOL metrology system
13.2.1 Model Eye
An IOL mounted within a wet cell and artificial cornea comprised the model eye. The ISO 11979–2 recommends the use of an aberration-free aspheric achromatic doublet to serve as the artificial cornea. The model eye in this system is an off-the-shelf 40.0D achromatic doublet (Edmund Optics). The wet cell within which the IOL was mounted consisted of a fluid-filled (balanced salt solution) compartment between two optically flat windows. The spacing between the wet cell and artificial cornea was set such that the ratio of the pupil diameter to the beam size at the IOL plane is in accordance with the Gullstrand model eye. A pupil camera was used to assure proper alignment of the IOL within the wet cell.
Three presbyopia-correcting IOLs were included in this study: Crystalens HD500 (Bausch & Lomb), ReSTOR +3D SN6AD1 multifocal (Alcon Laboratories, Inc.), and Tecnis ZM900 multifocal (Abbott Medical Optics, Inc.). For comparison, a monofocal IOL (AcrySof SN60AT, Alcon Laboratories, Inc.) was also included in the study.
13.2.2 Adaptive Optics
The pupil plane of the model eye was relayed to a large-stroke deformable mirror (Mirao 52D, Imagine Eyes) [2] using a pair of achromatic lenses (not shown in Fig. 13.1). By changing the surface profile of the deformable mirror, aberrations are induced in the model eye, allowing for the simulation of various corneal topographies. A custom-made Shack-Hartmann wavefront sensor was used to verify the aberration profiles generated by the deformable mirror.
13.2.3 Capturing the Retinal Image
The retinal image formed by the model eye was magnified with a microscope objective for adequate sampling with a charge-coupled device (CCD). Retinal images of the resolution target in white light (tumbling letter “E” acuity chart) were presented by a computer projector (PG-M20X, Sharp) placed in the retinal plane. A Badal optometer was used to simulate through-focus vergences and correct for defocus. A Badal optometer is the ideal method to induce various target vergences for through-focus image capture. Due to the telecentricity of a Badal optometer, the magnification of the retinal image is static relative to target vergence.
13.2.4 Quantifying Through-Focus Image Quality
There are several widespread techniques for quantifying optical quality of IOLs in an optical bench. A routine approach is measurement of the modulation transfer function (MTF). The MTF represents an optical system’s contrast attenuation for individual spatial frequencies (i.e., contrast in the image plane of an object with 100 % contrast). The MTF can be calculated by Fourier transforming the captured image of a point source or by directly measuring the contrast of gratings from a resolution target, such as the commonly used US Air Force resolution target.
In the presence of corneal lower and higher order aberrations, the eye’s point spread function is radially asymmetric and may be highly directional. Therefore, measuring optical quality in only two meridians (e.g., vertical and horizontal) may not be representative of overall image quality in all directions. We employed a technique based on the 2-dimensional spectral analysis of the retinal image. The benefit of this technique is that it quantifies overall image quality regardless of the orientation of the point spread function relative to the resolution target.
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